Fix read-only sub-resource CREST API descr (#6)

Corrects three obscure defects in the Rest2LDAP implementation of CREST descriptors:
- Read-only sub-resources were not appearing at all in the CREST API description JSON.
- When read-only sub-resources appeared in the API description alongside writable sub-resources for the same models, the generated service name for the sub-resources was the same, leading to an IllegalStateException. Now, we generate a unique service name for each sub-resource based on its writability.
- A top-level `create` request was still being rendered in the API description for read-only sub-resources.